I constantly look through the Friends section of the pause menu
in GTA V and notice that on several save files, people have what
looks like a fireman's helmet on Michael? But it's not the
firefighter outfit (as seen in the Bureau raid), they're wearing it
independently (often with the gray suit). None of them have replied
when I asked how they got it and I couldn't find anything regarding
it on this wiki. Is it just a glitch, or is there legitimately a
way to obtain it?

I only ask because I know GTA V is full of secret missions
(Children of the Mountain, The Last One, the Epsilon series).